Fluidized bed silicon deposition from silane

ABSTRACT

A process and apparatus for thermally decomposing silicon containing gas for deposition on fluidized nucleating silicon seed particles is disclosed. 
     Silicon seed particles are produced in a secondary fluidized reactor by thermal decomposition of a silicon containing gas. The thermally produced silicon seed particles are then introduced into a primary fluidized bed reactor to form a fludized bed. Silicon containing gas is introduced into the primary reactor where it is thermally decomposed and deposited on the fluidized silicon seed particles. Silicon seed particles having the desired amount of thermally decomposed silicon product thereon are removed from the primary fluidized reactor as ultra pure silicon product. 
     An apparatus for carrying out this process is also disclosed.

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Sections 305 of theNational Aeronautics and Space Act of 1958, public law 83-568 (72Statute 435; 42 U.S.C. 2454).

This is a division, of application Ser. No. 126,324, filed Mar. 3, 1980,now U.S. Pat. No. 4,314,525.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the production of silicon and moreparticularly to an improved system for the production of solar gradesilicon by the thermal decomposition of silane.

2. Description of the Prior Art

Self sufficiency in energy is a stated national goal. Most of theproposed means to achieve this goal are either environmentallyunacceptable or are not feasible, especially those not depending onfossil fuel sources. Of the available alternatives, solar energy is themost abundant, inexhaustable single resource available. However,capturing and utilizing solar energy is not simple. Methods are beingsought to convert solar energy to a concentrated, storable form ofenergy. A known method, photosynthesis, converts somewhat less than 1%of the suns energy at the earths surface to a solid fuel, i.e., plantmaterials, which when accumulated and transformed over geologic agesyielded fossil fuels. Current rates of use of these fossil fuels, andthe particular geographic distribution and political control of majorpetroleum resources pose problems for nations that are net petroleumconsumers. An alternate method yielding a simpler fuel, at a higherconversion, has long been desired.

One method of converting solar energy to a usable form being prominentlyconsidered is the deployment of large arrays of photovoltaic solarcells, especially in the sunbelt areas such as the southwestern andwestern regions of the United States. The most promising candidate forthe solar cell is a doped silicon sheet material and silicon is one ofthe most plentiful elements in the earth's crust.

The most abundant source of silicon is silica sand. Metallurgicalsilicon can be made cheaply from sand in an arc furnace by reaction withcarbon to produce silicon and CO₂. This material, however, is far tooimpure for use in solar cells. To purify this material, the silicon canbe converted to a gaseous product where distillation and adsorption canbe used for purification. For example, metallurgical silicon will reactwith chlorine to produce SiCl₄ gas. This gas can be converted to siliconusing a reductant such as zinc, as studied by Battelle MemorialInstitute, or to silane gas, SiH₄, by a redistribution reaction withhydrogen. This highly pure silane gas can then be thermally decomposedto silicon and hydrogen.

The thermal decomposition of highly pure silane gas to produce siliconis preferred over the formation of silicon from SiCl₄. The silanethermal decomposition reaction is preferred due to its simplicity,potential purity (no zinc or other reactant), and the relatively mildconditions of reaction.

A major problem present in producing high purity silicon by thermallydecomposing chlorosilane gas is that known processes include inherentlywasteful or inefficient energy consumption steps. For example, theSiemens process for the production of high purity silicon on heated rodsurfaces has inherent massive heat losses.

The current energy crisis has especially focussed attention on the majordeficiencies of these conventional chlorosilane thermal decompositionprocesses, where huge expenditures of energy are required per pound ofproduct. The need for new processes to produce high purity silicon ingreat volume, high efficiency or yield and at very low overall energyexpenditure levels is expected to become more critical as the worldturns to more widespread use of silicon solar energy collection systems.

An approach which has been taken to overcome the inherent energy wastepresent in prior processes utilizes a fluidized bed reactor (FBR) forthermally decomposing silane. Fluidized bed reactors are well known fortheir excellent heat and mass transfer characteristics.

Examples of fluidized bed reactor utilization to thermally decomposesilane to form silicon are disclosed in U.S. Pat. Nos. 3,012,861 and3,012,862.

These patents disclose use of a fluidized bed of ultra pure silicon seedparticles on which silicon is deposited during thermal decomposition ofsilane, chlorosilanes or other halo silanes within the fluidized bed.The silanes are introduced along with preheated fluidizing gas, such ashydrogen or helium into the fluidized bed reactor. Heat for the silanedecomposition reaction is supplied by conventional external heatingelements in conjunction with the preheated gas. The fluidizing gascontaining the silanes maintains the bed of pure silicon seed particlesin a fluidized state. The silanes present in the fluidizing gasthermally decompose within the fluidized bed reactor and are depositedon the seed crystals as opposed to the reactor side walls or othersurfaces.

As the silicon seed particles grow within the fluidized bed, the heavierparticles migrate to the lower portion of the fluidized bed where theyare removed for further processing as a highly pure silicon product.This method provides a relatively energy effecient process for producingthe high purity silicon required in solar arrays.

The continuous removal of silicon seed particles after they have grownto the desired size results in the depletion of the fluidized bed. Inorder to maintain a continuous bed of silicon seed particles within thereactor, it is necessary to continually introduce additional siliconseed particles into the fluidized bed to replace those removed as partof the ultra pure silicon product.

This necessity of providing a continuous supply of ultra pure siliconseed particles to the fluidized bed reactor presents a serious problemto which no adequate solution has yet been found. For example, methodsutilized for producing silicon seed bed particles in the siliconfluidized bed processes being developed recently involve one of twotechniques or a combination thereof. One of these processes involvescrushing, classifying and cleaning ultra pure silicon into appropriatelysized silicon seed particles. A hammer mill as well as jaw, cone orroller crushers are utilized to reduce the bulk silicon to a specificparticle size distribution suitable for use as seed particles. Thecrushing and classifying process is not only expensive and timeconsuming but also presents severe contamination problems. In additionthe crushing produces a highly acicular seed particle which presents anundesirable surface for efficient silicon deposition. Procedures toprocess the acicular seed particles into a more uniform shape involvedry tumbling or wet tumbling in the presence of water or methanol.Possiblities for contamination and long tumbling times render this typeof procedure undesirable.

The other process for producing silicon seed particles is not plaguedwith the above mentioned problems inherent in crushing and grindingprocesses, but it also presents significant problems of its own. Thisprocess involves the recycling of appropriately sized seed particleswhich are generated in the fluidized bed reactor and removed along withthe larger silicon product particles or entrained overhead in fluidizinggas exiting the reactor. In the reactor, a certain amount of siliconcrystallizes spontaneously to form so-called homogeneous particles,while the majority of the silicon formed through thermal decompositionundergoes heterogeneous crystallization on the surface of the seedsilicon particles. These homogeneous silicon particles form the bulk ofthe silicon particles recycled back into the reactor as seed particles.

In general, there is not a sufficient amount of appropriately sizedhomogeneous particles produced during normal reactor operations tosupply the entire demand for new seed particles in the fluidized bed.This requires that the recycled stream of homogeneous particles besupplemented by seed particles produced by crushing or grinding. Inaddition, and more importantly, these homogeneously formed siliconparticles tend to be amorphorous or not-dense in nature and present avery undesirable surface for elemental silicon deposition.

It is apparent that there is a present need for a method of providing acontinuous supply of appropriately sized suitable silicon seed particleswithout the problems inherent in the above two discussed processes.Until a convenient, economical and suitably pure source of silicon seedparticles is found, the fluidized bed process for producing ultra puresilicon will be less than optimally desirable.

SUMMARY OF THE INVENTION

The present invention solves the above-mentioned problems of prior artsilicon producing processes by providing a novel seed source means forsupplying silicon seed particles to the fluidized reactor.

This novel seed source means includes a seed reactor for thermallydecomposing a seed generating gas containing silicon to form precursorsilicon seed particles. The seed reactor foregoes the need forundesirable crushing and grinding steps and additionally producessilicon seed particles which are uniformly shaped for desirablefluidization characteristics and elemental silicon deposition.

In general, the present invention comprises a primary fluidized bedreactor in combination with a silicon seed generating device. Means areprovided for introducing silicon seed particles into the primaryfluidized reactor zone. Means are also provided for introducing afluidizing gas and a silicon containing gas into the primary fluidizedreactor zone to maintain the silicon seed particles in a fluidizedsuspension. Means are also supplied for heating the primary fluidizedreactor zone to a temperature sufficient to thermally decompose thesilicon in the silicon-containing gas to elemental silicon fordeposition on the silicon seed particles. In addition means are includedfor removing excess fluidizing gas, unreacted silicon containing gas,reaction by-product gases and any entrained silicon particles thereinfrom the primary fluidized reactor zone. Finally means are provided forremoving silicon seed particles having thermally decomposed siliconproducts thereon from the primary fluidized reactor zone.

As the silicon seed particles having thermally decomposed siliconproducts thereon are removed from the primary fluidized reactor zone,the amount of silicon seed particles present is depleted and must becontinuously replenished. The present invention provides a novel seedsource means for supplying new silicon seed particles to the primaryfluidized reactor zone to replace seed particles removed as product. Theseed source means of the present invention is also based on thermaldecomposition of silicon containing gases and therefore bypasses theundesirable crushing and grinding steps inherent in prior art methodsfor producing silicon seed particles. In addition, the silicon seedparticles produced by the thermal decomposition process of the presentinvention does not produce undesirable porous silicon seed particles orseed particles having acicular properties such as those found in siliconseed particles produced by prior art methods.

These and many other features and attendant advantages of the presentinvention will become apparent as the invention becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a first preferred exemplaryembodiment of the present invention.

FIG. 2 is a diagrammatic representation of a second preferred exemplaryembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EXEMPLARY EMBODIMENTS

The present invention deals with a new method and apparatus forproducing silicon seed particles for introduction into a fluidized bedreactor utilized in producing ultra-pure silicon. As shown in FIGS. 1and 2, there are two preferred apparatuses and methods for thermallyproducing the silicon seed particles. The primary fluidized bed reactorwhich receives the silicon seed particles from either of the twopreferred embodiments of the present invention is the same for bothembodiments. Therefore, the primary fluidized bed reactor will bediscussed generally followed by a detailed discussion of each of the twopreferred embodiments of the present invention for supplying siliconseed particles to the primary fluidized bed reactor.

A primary fluidized bed reactor is shown generally at 10 in both FIGS. 1and 2. The primary fluidized bed reaction 10 is of the type known in theart of ultra-pure silicon production by thermal deposition of elementalsilicon on fluidized silicon seed particles. The actual dimensions andoperating parameters of the primary fluidized bed reactor 10 are notcritical and, in general, are known in the art.

For example, the primary fluidized bed reactor 10 includes a reactorshell 12 which defines a primary fluidized reactor zone 14. The reactorzone dimensions may vary from a cylinder having a diameter of betweenabout 1/4 inch to 4 feet and a heighth of between about 6 inches and 9feet. The reactor shell 12 may be composed of any suitable inertmaterial such as stainless steel, silicon carbide and preferably quartz.The reactor shell material should be inert at the temperatures ofsilicon decomposition (i.e. 400° to 1200° C.).

Means for introducing silicon seed particles into the primary fluidizedreactor zone 14 is provided by silicon seed particle inlet 16. Thesilicon seed particles which are introduced through the seed particleinlet into the primary fluidized bed reactor zone 14 are maintained in afluidized suspension by appropriate fluidizing gas. Means forintroducing the fluidizing gas into the fluidized reactor zone 14 isprovided by a plurality of upwardly directed gas inlets 18 provided inthe sidewall of reactor 10. Of course, if desired the fluidizing gas maybe introduced through less gas inlets and in a different position, suchas only from the bottom; however, it is believed that a plurality of gasinlets 18 may increase the uniform fluidization of silicon seedparticles and expand the reaction zone as the reaction rate is generallyvery fast.

Means for heating the primary fluidized reactor zone 14 are provided byheating coils 20. The heating coils 20 can comprise any convenient heatsource known in the art for externally heating the primary fluidizedreactor zone 14 to temperatures of silicon containing gas decomposition.It is important that the temperature of the primary fluidized reactorzone 14 be maintained at a temperature above the silicon decompositionpoint while still remaining below the temperature at which silicon meltsto form a liquid. This temperature range is generally between about 400°to 1200° C. for a silane system.

Gas inlets 18 can also provide a means for introducing siliconcontaining gas into the primary fluidized reactor zone 14. If desiredthe silicon containing gas can be introduced into the primary fluidizedreactor zone 14 by way of gas inlets separate from the fluidizing gasinlet 18; however, it is preferred to premix the silicon containing gaswith the inert fluidizing gas and introduce the gas mixture into thefirst fluidized bed reactor by way of gas inlets 18. The siliconcontaining gas may be any of the number of thermally decomposablesilicon containing gases such as silicon halides or silane. In thepresent invention, the use of silane is preferred become it foregoes theneed of catalysts utilized in thermal decomposition of many othersilicon containing gases. The fluidizing gas may be any inert gas suchas hydrogen or helium; however, hydrogen is preferred. Preferably, thefluidizing gas should contain between about 5% to 100% silane by volumewhen introduced through inlets 18 into the primary fluidized reactorzone 14.

Means are provided by outlet 22 for removing excess fluidizing gas,unreacted silane, reaction by-product gases including hydrogen and anyentrained silicon particles therein from the primary fluidized reactorzone 14. During thermal decomposition of the silane gas within theprimary fluidized reactor zone 14, a certain amount of fine silicon dustis produced. These fine silicon particles are entrained in the exhaustgases and removed via outlet 22 and transferred to a cyclone separator24. The cyclone separator vents the exhaust gases as shown by arrow 26and recycles the fine silicon particles ranging in size fromapproximately a micron up to more than 50 microns back into the primaryfluidized reactor zone 14 through recycle conduit 28 and inlet conduit30.

Means are provided by product outlet 32 for removing silicon productparticles from the primary fluidized reactor zone 14. The continuallygrowing silicon seed particles migrate towards the bottom of the primaryfluidized reactor zone 14 and eventually become so large that they areless fluidizable and thus segregated from the bottom of the reactor.These less fluidizable silicon particles having elemental siliconthermally deposited thereon are then removed from the bottom of theprimary fluidized reactor zone via product outlet 32.

The continual removal of silicon particles from the primary fluidizedbed reactor zone 14 results in a gradual depletion of silicon particleswithin the primary fluidized reactor zone 14. It is essential tocontinuous operation of this fluidized bed process that new silicon seedparticles be continually introduced into the primary fluidized bedreactor zone 14. It is to the two preferred embodiments of the presentinvention for producing such silicon seed particles that the remainderof the detailed description is directed.

The first preferred seed source means for supplying silicon seedparticles to the fluidized bed reactor 10 is shown in FIG. 1 comprisingtwo basic elements. These elements are the pyrolysis seed reactor orfree space reactor shown generally at 34 and separator 36. The basicfunction of the pyrolysis seed reactor 34 is to thermally decompose asilicon containing gas to homogeneously and heterogeneously formprecursor silicon seed particles ranging in diameter from 0.1 microns toin excess of 50 microns. The basic function of separator 36 is toseparate and classify these precursor silicon seed particles intoparticles having a diameter greater than or equal to 50 microns andparticles having diameters less than 50 microns. The precursor siliconseed particles having diameters greater than or equal to 50 microns arepassed via inlet conduit 30 to the first fluidized reactor zone 14. Thesilicon particles having diameters less than 50 microns are recycledback to the pyrolyzer seed reactor 34 by conduit 38. Prior toreintroduction into the pyrolysis seed reactor 34, the silicon particlesare separated from the gas in which they are entrained by particlefeeder-separator 40. The separated off gases are vented as shown byarrow 42. The amount of silicon particles recycled back into thepyrolysis seed reactor is controlled by valve 44.

The pyrolysis seed reactor 34 includes pyrolyzer shell 46 which definesthe pyrolysis seed reactor zone 48. The pyrolyzer shell 46 may becomposed of a variety of heat resistant materials; however 1/4"stainless steel is preferred. Means are provided by inlets 50 and 52 forintroducing a seed generating gas containing silicon into the pyrolyzerseed reactor zone 48. The silicon containing gas can be any one of thethermally decomposable types including the silicon halides and silane.Silane is preferred. The seed generating gas may be diluted with up to95% of an inert gas preferably hydrogen. The usual composition of theseed generating gas is from between about 5 to 100% silane by volume inhydrogen gas.

Means for heating the pyrolysis seed reactor zone 48 is provided byporous thermowell shown generally at 54. The porous thermowell 54 is anelongated heating element which is disposed internally within thepyrolysis seed reactor zone 48. The porous thermowell 54 may be heatedin any number of conventional ways including electrical resistanceheaters.

Preferrably the thermowell 54 includes a porous carbon cylinder 53defining a central distributor space 49 inside which is disposed adouble helix heating element 55. The heating element is preferrablycomposed of silicon carbide and is provided with leads 51 for connectionto an appropriate power source. The porous thermowall 54 must be capableof heating the pyrolysis seed reactor zone to the desired thermaldecomposition of silicon, temperatures generally in the range of 400° C.to 1200° C.

An important aspect of the porous thermowell 54 is that the poroussurface of the thermowell must be constructed to allow an inertprotective gas to be passed outwardly from the central distributor space49 through the porous carbon cylinder 53. This outwardly passing inertgas provides a protective boundary adjacent the exterior surface of thethermowell 54 to prevent the contact of silicon or silane with theporous thermowell 54 surface. If the silicon or silane is allowed tocontact the hot thermowell 54 surface, continuing silicon depositioncould occur on the thermowell 54 thereby eventually rendering thepyrolysis seed reactor 34 useless. Preferrably, the inert gas is passedinto the central distributor space 49 by way of conduit 47. The inertgas is dispersed uniformly throughout tne central distributor space 49and passes outwardly through the porous carbon cylinder 53 to form auniform protective gas boundary layer surrounding the carbon cylinder53. Helium is the preferred gas due to its high heat capacity. As thehelium is passed through the porous thermowell 54 and into the reactorzone 48, it carries heat with it to help provide uniform heating withinthe reactor zone 48.

The problem of silane or silicon contacting the hot thermowell surfaceis also presented with regards to the pyrolyzer shell 46. Accordingly,the interior surface of the pyrolyzer shell 46 is provided with a poroussurface 56. The particular porous material which is used to form theporous surface 56 is not particularly critical, so long as, the materialis refractory in nature and maintains its porous structure at elevatedtemperatures and does not affect the silane decomposition reaction.

Porous surface 56 is preferrably a fine mesh stainless steel screenthrough which an inert gas, preferrably argon is passed uniformly intothe reactor zone 48 to provide a protective inert gas boundary. Outerwall distributor space 57 is provided between the pyrolyzer shell 46 andthe porous surface 56 to permit uniform distribution of the inert gas tothe porous surface 56. Inert gas inlets 59 are centrally located in thesidewall of the pyrolyzer shell 46 to provide inert gas to thedistributor space 57. Of course, if desired a plurality of inert gasinlets can be used instead of just the two inlets 59 shown in thisembodiment.

The precursor silicon particles produced in the pyrolysis seed reactorzone 48 are removed from the bottom via line 58 and transferred toseparator 36 where the above mentioned separation and classificationtakes place. The actual dimensions of the pyrolysis seed reactor 34 arenot especially critical. In addition operating parameters such as silaneand hydrogen gas flow rates, protective inert gas flow rates andoperating temperatures can be established experimentally with differentpyrolysis seed reactors having different sizes and configurations.

Preferrably the pyrolysis seed reactor 34 is operated at as low atemperature as possible, for example 600°. Also smaller reactors arepreferred. Adequate silicon production has been achieved using apyrolysis seed reactor having an internal diameter of 5 inches and alength of 35 inches. Additionally, although the pyrolyzer seed reactor34 and separator 36 are shown supplying silicon seed particles to onlyone primary fluidized bed reactor, it should be understood that aplurality of primary fluidized bed reactors could also be supplied withsilicon seed particles from one pyrolysis seed reactor source.

The second preferred embodiment of the present invention is shown inFIG. 2. The same primary fluidized bed reactor 10 as was used in thefirst preferred embodiment is also used in the second preferredembodiment. The seed source means of the second preferred embodiment forsupplying silicon seed particles to the primary fluidized bed reactorzone includes two basic elements. The primary element is the pyrolysisseed reactor shown generally at 134. The second basic element is thesecondary fluidized reactor shown generally at 60. The pyrolyzer seedreactor 134 is similar to, if not identical, with the pyrolyzer seedreactor 34 of the first preferred embodiment. However, in this secondpreferred embodiment the second fluidized bed reactor 60 is substitutedfor separator 36.

Many of the silicon particles which are produced in the pyrolysis seedreactor 134 are very small, on the order of 0.1 microns to 10 microns.This makes it very difficult to fluidize them efficiently in a primaryfluidized bed reactor. The secondary fluidized bed reactor 60 istherefore utilized as an intermediate fine particle fluidized bedreactor to increase the size range of the small silicon particles inorder to feed the primary reactor. As was the case in the firstpreferred embodiment, the pyrolysis seed reactor 134 of the secondpreferred embodiment includes a pyrolyzer shell 146 defining a pyrolysisseed reactor zone 148. Silane and inert gas inlets are provided at 150and 152. The pyrolysis seed reactor zone 148 is also heated by porousthermowell 154.

The thermowell 154 similarly includes a porous carbon cylinder 153defining a central distributor space 149 inside which is disposed adouble helix heating element 155. Leads 151 are provided for connectingthe heating element 155 to an appropriate power source. The pyrolyzershell 146 is also supplied with interior porous surface 156.

The porous surface 156 is again preferrably a fine mesh stainless steelscreen which is placed inwardly from the pyrolyzer shell 146 so as toleave an outer wall distributor space 157 therebetween. As with thefirst embodiment, inert argon gas is preferrably passed through inlets159 into the wall distributor space 157 and uniformly passed through theporous surface 156 into the reactor zone 148 to provide the desiredinert gas protective boundary adjacent the porous surface 156. Likewise,helium is passed by way of conduit 147 into the central distributorspace and passed outwardly through the porous carbon cylinder 153 toform the inert gas protective boundary surrounding the thermowell 154.

The precursor silicon seed particles are also passed out of thepyrolysis seed reactor zone 148 by way of outlet 158. Since in thispreferred embodiment the precursor silicon seed particles are passed tothe secondary fluidized bed reactor 60, the conditions within thepyrolysis seed reactor 134 may be reduced so that precursor silicon seedparticles measuring between 0.1 microns and 5 microns in diameter areproduced. These relatively small precursor silicon seed particles arethen introduced into the secondary fluidized bed seed reactor 60 asshown at inlet 62. The purpose of the secondary fluidized bed reactor 60is to thermally deposit elemental silicon on these relatively small(less than 5 microns) particles to produce larger appropriately sizedsilicon seed particles having diameters of 50 microns and above.

The secondary fluidized bed reactor 60 includes a reactor shell 64 whichdefines a secondary fluidized reactor zone 66. Means for introducingsilane and inert gas are provided by inlet 68. Means for heating thesecondary fluidized reactor zone 66 are provided by external heatingcoils 70. In general, the secondary fluidized bed reactor 60 will havedimensions smaller than the primary fluidized reactor. Due to the factthat the very small silicon particles are difficult to fluidize, specialflow conditions are required in the secondary fluidized reactor zone 66.Hydrogen is the preferred fluidizing gas and silane is the preferredsilicon containing gas. These two gases are preferrably pre-mixed priorto entry into the secondary fluidized reactor zone 66. Preferably thecomposition of the combined silane and inert gas ranges from 5 to 100%silane by volume with hydrogen gas.

Means are also provided by way of outlet 72 for removing excess hydrogenfluidizing gas, unreacted silane, reaction by-product gases includinghydrogen and any entrained silicon particles therein from the secondaryfluidized reactor zone 66. These exhaust gases and entrained siliconparticles are passed to cyclone separator 74. The cyclone separator 74separates out the entrained silicon particles ranging in diameter from0.1 to 5 microns and reintroduces them via line 76 to line 158 forintroduction back into the secondary fluidized reactor zone 66. Theoverhead exhaust gases which may still contain some ultra-fine siliconparticles are then passed out of the cyclone separator 74 through line78 to the particle feeder-separator 140. As was the case in the firstpreferred embodiment, the particle-feeder separator 140 vents theexhaust gases through vent 142 and returns the ultra-fine (diametersless than 0.1 micron) particles to the pyrolyzer seed reactor zone 148by way of valve 144.

Means are also provided by outlet 80 to remove precursor silicon seedparticles having thermally decomposed silicon thereon from the secondaryfluidized reactor zone 66. These particles have a diameter which isgenerally greater than or equal to 50 microns. These particles are thentransferred via line 82 into the primary fluidized reactor zone 14. Theyare for the most part spherical in shape, non-pourous and provide a verydesirable surface for deposition of elemental silicon from thermaldecomposition of silane.

The actual dimensions, flow rates and operating temperatures are alsonot especially critical with regard to the secondary fluidized bedreactor 60. They may also be established according to the size of thesecondary fluidized bed reactor 60 desired and according to differingconfigurations.

Having thus described two preferred embodiments of the present inventionwhich disclose a new and improved seed source means for supplyingsilicon seed particles to a fluidized bed silicon reactor, it isapparent that obvious advantages are provided over prior art grindingand crushing techniques. For example, contamination problems inherent ingrinding and crushing processes are eliminated. A spherical non-porousseed particle is also produced which does away with problems inherent inacicular particles and porous particles. Additionally, this new processfor supplying silicon seed particles maximizes the use of silane gas byrecycling even the minutest silicon particles.

Further, it should be noted by those skilled in the art that the withindisclosures are exemplary only and that various other alternatives,adaptations and modifications may be made within the scope of thepresent invention. Accordingly the present invention is not limited tothe specific embodiments as illustrated herein.

What is claimed is:
 1. A process for producing silicon comprising thesteps of:introducing fluidizing gas into a primary fluidized reactorzone in an amount sufficient to maintain a bed of silicon seed particlesin a fluidized introducing a silicon containing gas capable of gas phasethermal decomposition into said primary fluidized reactor zone; heatingsaid primary fluidized reactor zone to a temperature sufficient tothermally decompose the silicon in said silicon containing gas toelemental silicon but not above the melting point temperature ofsilicon; depositing the elemental silicon formed by said thermallydecomposed silicon containing gas on said suspended seed particles;continuously removing said seed particles with elemental silicondeposited thereon from said primary fluidized reactor as siliconproduct; removing excess fluidizing gas, unreacted silicon containinggas, reaction by-product gases and any entrained silicon particlestherein from said primary fluidized reactor zone; continuouslyreintroducing silicon seed particles into said primary fluidized reactorzone to replace silicon seed particles which are removed as siliconproduct; and producing a continuous supply of said silicon seedparticles for introduction into said primary fluidized reactor zone bythermally decomposing a seed generating gas containing silicon in apyrolysis seed reactor zone to form precursor seed particles includingthe steps of; introducing said seed generating gas into said seedreactor zone; heating said pyrolysis seed reactor to a temperaturesufficient to thermally decompose the silicon in said seed generatinggas to form elemental silicon, but not above the melting pointtemperature of silicon; passing a protective inert gas inwardly throughporous reactor walls to form a protective inert gas boundary to preventcontact of silicon with the porous reactor walls; thermally decomposingthe silicon in said seed generating gas to form precursor silicon seedparticles; removing said precursor silicon seed particles from saidpyrolysis seed reactor zone; separating said precursor silicon seedparticles into larger silicon seed particles for introducing into saidprimary fluidged reactor zone and smaller silicon particles which arerecycled back to said pyrolysis seed reactor zone for further growth andintroducing said separated larger precursor silicon seed particles intosaid primary fluidized reactor zone as the silicon seed particles. 2.The process of claim 1 wherein said precursor silicon seed particles areseparated into seed particles having diameters substantially greaterthan 50 microns and recycle particles having diameters less than 50microns.
 3. The process of claim 1 which further includes the stepsof:introducing said precursor silicon seed particles into a secondaryfluidized reactor zone; introducing fluidizing gas into said secondfluidized reactor zone in an amount sufficient to maintain saidprecursor silicon seed particles in fluidized suspension; introducing asilicon containing gas capable of gas phase thermal decomposition intosaid secondary fluidized reactor zone; heating said secondary fluidizedreactor zone to a temperature sufficient to thermally decompose thesilicon in said silicon containing gas to elemental silicon, but notabove the melting point temperature of silicon; depositing the elementalsilicon formed by said thermally decomposed silicon containing gas onsaid suspended precursor seed particles; continuously removing saidprecursor silicon seed particles with elemental silicon depositedthereon from said secondary fluidized reactor zone and transporting themto said primary fluidized reactor zone for introduction as said siliconseed particles; and removing excess fluidizing gas, unreacted siliconcontaining gas, reaction by-product gases and any entrained siliconparticles therein from said secondary fluidized reactor zone.
 4. Theprocess of claim 3 which further includes the step of separating saidentrained silicon particles from said excess fluidizing gas, unreactedsilicon containing gas and reaction by-product gases and recycling saidseparated out entrained silicon particles back into said secondaryfluidized reactor zone.
 5. The process of claim 1 wherein said precursorsilicon seed particles removed from said pyrolysis seed reactor zonehave diameters between 0.1 and 5 microns.
 6. The process of claim 1wherein the heating of said pyrolyzer seed reactor zone is provided by aheating element disposed internally within said pyrolysis seed reactorzone wherein silicon is prevented from contacting said heating elementby passing an inert gas outwardly through a porous layer encompassingsaid heating element to provide a protective inert gas boundarysurrounding said heating element.
 7. The process of claim 6 whichadditionally includes the step of separating said precursor silicon seedparticles into silicon seed particles for introduction into said primaryfluidized reactor zone and smaller silicon particles which are recycledback to said pyrolysis seed reactor zone for further growth.
 8. Theprocess of claim 6 which additionally includes the steps of:introducingsaid precursor silicon seed particles into a secondary fluidized reactorzone; introducing fluidizing gas into said second fluidized reactor zonein an amount sufficient to maintain said precursor silicon seedparticles in fluidized suspension; introducing a silicon containing gascapable of gas phase thermal decomposition into said second fluidizedreactor zone; heating said second fluidized reactor zone to atemperature sufficient to thermally decompose the silicon in saidsilicon containing gas to elemental silicon, but not above the meltingpoint temperature of silicon; depositing the elemental formed by saidthermally decomposed silicon containing gas on said suspended precursorseed particles; continuously removing said precursor silicon seedparticles with elemental silicon deposited thereon from said secondaryfluidized reactor zone and transporting them to said primary fluidizedreactor zone for introduction as said silicon seed particles; andremoving excess fluidizing gas, unreacted silicon containing gas,reaction by-product gases and any entrained silicon particles thereinfrom said secondary fluidized reactor zone.
 9. The process of claim 1wherein the entrained particles removed from said primary fluidizedreactor zone are separated from the excess fluidizing gas, unreactedsilicon containing gas and reaction by-product gases and reintroducedback into said primary fluidized reactor zone.
 10. The process of claim1 wherein the fluidizing gas is introduced into said primary fluidizedreactor zone through a plurality of gas inlets.
 11. The process of claim1 wherein said fluidizing gas and said gas containing silicon are mixedprior to introduction into said primary fluidized reactor zone.
 12. Theprocess of claim 3 wherein said fluidizing gas and said gas containingsilicon are mixed prior to entry into said secondary fluidized reactorzone.
 13. The process of claim 11 wherein said fluidizing gas beingintroduced into said primary fluidized reactor zone contains betweenabout 5% and 100% by volume silicon containing gas.
 14. The process ofclaim 12 wherein said fluidizing gas being introduced into saidsecondary fluidized reactor zone contains between about 5% and 100% byvolume silicon containing gas.
 15. The process according to claim 1wherein said silicon containing gas is silane.
 16. The process accordingto claim 1 wherein said seed generating gas contains from between about5% to 100% by volume of a silicon containing gas.
 17. The processaccording to claim 1 wherein said silicon containing gas is silane. 18.The process of claim 1 wherein said primary fluidized reactor zone isheated to between about 400° to 1200° C.
 19. The process of claim 1wherein said pyrolysis seed reactor zone is heated to a temperature ofbetween about 400° and 1200° C.
 20. The process of claim 3 wherein saidsecondary fluidized reactor zone is heated to a temperature of betweenabout 400° and 1200° C.
 21. A process according to claim 13 in which theseed reactor zone is heated by a heating element disposed centrally andaxially in said zone.